1. Field of the Invention
This invention relates to a method of locating defects in underground conduits, and in particular to locating leaks in steam pipes buried in noisy environments, determining the rate and direction of flow within a conduit, and locating defects in electrical conduits.
2. Description of Related Art
A. Introduction PA1 B. Characteristics of Steam Conduits PA1 C. Needs
Conduits for transmission are often buried to protect them, and to save space. This is common in and around cities for the transmission of water and steam, around industrial plants for the transmission of chemicals or fuels, or even across the country for the transmission of natural gas and electric power. Leaks in any of these conduits can be costly due to the loss of the transmitted fluid and dangerous because of the accumulation of toxic or explosive fluids outside the conduit.
Responsible practice therefore requires the detection of a leak, its precise location, and its repair. The location of the leak is most important in crowded environments due to the disruption caused by excavation. One method to detect the leak is to use surface sensor techniques which are hindered by traffic, by turbulent flow within the conduit, by any discontinuities in the conduit such as joints and traps, or by noise generated by adjacent conduits. Another method is to drill "bare holes" down to the conduit. This can be dangerous to the integrity of the conduit and to repair personnel.
Common systems of leak detection include: acoustic emission, infrared spectroscopy, tracer gas, and electrical (Detection and Location of Leaks in District Heating Systems, D. S. Kupperman et al., Argonne National Laboratories, ANL 92/5, March 1992). Half of the users of acoustic technology feel that current acoustic methods are not as effective as they desire (ibid.).
Acoustic methods of detection are further complicated by the differing transmission properties of the conduit itself, typically a metal, and the medium being transmitted, either a gas or a fluid. Experience has shown that the acoustic energy released by a steam leak propagates down the metal conduit in three modes with velocities of about 2500, 4000, and 6000 feet per second for 16 inch or 24 inch diameter steam conduit. The frequencies propagating in the metal are strongest below 1000 Hz and are severely attenuated at higher frequencies. Steam conduits also contain thermal expansion joints, and the leak noise is not discernible across these joints.
The primary medium for acoustic leak location is therefore the steam, where propagation is unaffected by the expansion joints. The propagation is multimode, with the strongest amplitudes propagating with velocities about 500 and 1000 feet per second, and with least attenuation between 1000 and 8500 Hz. The leak noise attenuates at a rate between 0.07 and 0.15 dB per foot between these frequencies. The movement of steam also creates a flow noise caused by discontinuities such as joints and by turbulence at higher flow rates. Experiment has shown that the flow noise amplitude is greatest below 2500 Hz.
A steam conduit is clearly not an ideal transmission line for the propagation of information. Solving the wave equation, which is well known in the communication art, predicts what modes might be supported by the conduit, but factors such as the location of the leak and the structural support for the conduit determine which modes actually propagate, and this differs at every site. The propagation velocity is therefore different at each site depending upon which modes propagate.
Accordingly, there is a need to accurately locate leaks in conduits, particularly where the cost of excavation is high and where the danger to property, the environment, and humans is great. Non-intrusive methods to determine the propagation velocity of leak noise, the medium flow rate, and its direction would enhance locating the leak. Extending the range of detection, particularly in low signal to noise ratio locations, would also improve the process.
3. Summary of the Invention
The present invention relates to a method to more accurately locate a leak in a conduit, particularly in a noisy environment. The present invention also relates to a method of determining the flow rate and direction of a medium in a conduit.
In one embodiment of the invention, sensors are attached to the conduit at three locations where they are separated by a known distance from each other. Noise from the steam leak propagating in the conduit is detected by the sensors and an electrical signal is generated at each location. The signals are recorded and converted into digital form to preserve them. Each signal is filtered to pass a frequency band from 4000 to 8500 Hz to discriminate against turbulent flow noise in the steam, noise transmitted by the conduit, and single frequency tones. A cross-correlation function from leak noise data obtained from a first pair of the three sensors located along the conduit is calculated to obtain a raw plot of a first time differential. In general, "time differential" refers to the raw cross-correlation function (plot), which measures the time delay for each mode of propagation, therefore, having many peaks A cross-correlation function is also calculated from leak noise data obtained from a second pair of the three sensors located along the conduit to get a raw plot of a second time differential. These plots are smoothed to obtain a peak time differential in each plot. "Peak time differential" refers to the peak obtained after smoothing the raw cross-correlation function. This peak refers to the delay used in the localization equations. The velocity of propagation for leak noise in the conduit is then calculated using the first peak time differential and the known spacing between the first pair of sensors. An uncorrected location of the leak is determined using the velocity of propagation, the second peak time differential, and the known spacing between the second pair of sensors. This location may be adjusted by considering the rate and direction of flow of a medium within the conduit to determine the final leak location as disclosed herein.
In alternative measurement of the invention, an acoustic method determines the flow rate and direction of steam flowing in a conduit. Sensors are attached to the conduit at two locations which are separated by a known distance. A vibration is imposed upon the conduit on one side of the sensors, and it is detected by both sensors. The process is repeated where a second vibration is imposed on the other side of the sensors. The signals generated by the sensors are recorded, in either analog or digital form, filtered to pass a frequency band from 4000 to 8500 Hz to discriminate against turbulent flow noise in the steam, noise transmitted by the conduit, and single frequency tones, and then a cross-correlation function from data obtained from the sensors from the first imposed vibration is calculated to obtain a raw plot of a first time differential, and the same is done for the second vibration. Each raw plot of time differential is smoothed to obtain a peak time differential in each plot. The center velocity of propagation is determined using the first peak time differential and the known spacing between the sensors, and the process is repeated for propagation in the other direction. The flow rate and direction of the medium in the conduit are then calculated from the difference in velocities.
In a further embodiment of the invention, a defect in an electrical conduit is located by imposing an electrical pulse upon the conductor with sufficient potential to cause an electric discharge at the defect. The acoustic energy liberated by the electric discharge is determined with two sensors, each mounted to the conduit and separated from the other along a length of the conduit which does not include the defect. Recording of the acoustic data can be synchronized to the leading edge of the pulse. The envelope of a cross-correlation function for each sensor signal is calculated to determine its peak which provides the time differential between the leading edge of the electrical pulse received at the first sensor and that received at the other sensor. The velocity of acoustic energy propagation is calculated from the time differential provided by a cross-correlation function of data from the two sensors and the known spacing between them. Inspection of the sign of the time differential determines which sensor is nearest to the defect. The location of the defect is computed knowing the velocity of propagation of the acoustic energy, the nearest sensor location, and the time of propagation to that sensor. The defects in this case may include an electrical leakage path in a solid dielectric or a leak in a fluid dielectric.
In still another embodiment of the invention, a defect in an electrical conduit is located by imposing an electrical pulse upon the conductor with sufficient potential to cause an electric discharge at the defect. The acoustic energy liberated by the electric discharge is determined by three sensors, each spaced from the others by a known distance along the conduit. The span between one pair of these sensors includes the location of the defect. Computing cross-correlation functions between data from pairs of sensors, and smoothing the plots to get peak time differentials provides a velocity of propagation of acoustic energy and the location of the defect. The defects in this case may include an electrical leakage path in a solid dielectric or a leak in a fluid dielectric.
The previously described versions of the present invention have many advantages including: the ability to better locate a leak in a buried conduit, to protect the environment and to reduce the cost and disruption of excavation; the ability to locate leaks in a noisy environments which are typical in cities, and to reject uninteresting sources of noise which include turbulent flow within the conduit, discontinuities in the conduit such as joints and traps, or noise generated by adjacent conduits; the ability to detect a leak at greater distances from the sensors; the ability to determine the direction and flow rate of a medium within a conduit in a non-intrusive manner; and the ability to locate a defect in an electrical transmission line.
These and other features and advantages of the invention will be better understood with consideration of the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.